Re: [PATCH v2 2/6] perf/core: Introduce percpu perf_cgroup

From: Peter Zijlstra
Date: Wed Mar 23 2022 - 08:51:31 EST


On Tue, Mar 22, 2022 at 08:08:30PM +0800, Chengming Zhou wrote:

> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 8b5cf2aedfe6..848a3bfa9513 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c

> @@ -843,11 +845,21 @@ static void perf_cgroup_switch(struct task_struct *task)
> */
> local_irq_save(flags);
>
> + cgrp = perf_cgroup_from_task(task, NULL);
> + if (cgrp == __this_cpu_read(cpu_perf_cgroup))
> + goto out;
> +
> + __this_cpu_write(cpu_perf_cgroup, cgrp);
> +
> list = this_cpu_ptr(&cgrp_cpuctx_list);
> list_for_each_entry_safe(cpuctx, tmp, list, cgrp_cpuctx_entry) {
> WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0);
>
> perf_ctx_lock(cpuctx, cpuctx->task_ctx);
> +
> + if (cpuctx->cgrp == cgrp)
> + continue;
> +
> perf_pmu_disable(cpuctx->ctx.pmu);
>
> cpu_ctx_sched_out(cpuctx, EVENT_ALL);

This is just straight up broken.. you can't continue after taking a
lock, that'll miss unlock.

Also, I really don't see the point of cpu_perf_cgroup, cpuctx->cgrp
should be able to do this.

Also, perhaps merge this in the previous patch, I'm not sure it makes
sense to split this.

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -833,6 +833,7 @@ static DEFINE_PER_CPU(struct list_head,
*/
static void perf_cgroup_switch(struct task_struct *task)
{
+ struct perf_cgroup *cgrp;
struct perf_cpu_context *cpuctx, *tmp;
struct list_head *list;
unsigned long flags;
@@ -843,11 +844,20 @@ static void perf_cgroup_switch(struct ta
*/
local_irq_save(flags);

+ cgrp = perf_cgroup_from_task(task, NULL);
+
list = this_cpu_ptr(&cgrp_cpuctx_list);
list_for_each_entry_safe(cpuctx, tmp, list, cgrp_cpuctx_entry) {
WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0);

+ if (READ_ONCE(cpuctx->cgrp == cgrp))
+ continue
+
perf_ctx_lock(cpuctx, cpuctx->task_ctx);
+
+ if (cpuctx->cgrp == cgrp)
+ goto next;
+
perf_pmu_disable(cpuctx->ctx.pmu);

cpu_ctx_sched_out(cpuctx, EVENT_ALL);
@@ -855,50 +865,22 @@ static void perf_cgroup_switch(struct ta
* must not be done before ctxswout due
* to event_filter_match() in event_sched_out()
*/
- cpuctx->cgrp = perf_cgroup_from_task(task,
- &cpuctx->ctx);
+ WRITE_ONCE(cpuctx->cgrp, cgrp);
/*
* set cgrp before ctxsw in to allow
* event_filter_match() to not have to pass
* task around
- * we pass the cpuctx->ctx to perf_cgroup_from_task()
- * because cgroup events are only per-cpu
*/
cpu_ctx_sched_in(cpuctx, EVENT_ALL, task);

perf_pmu_enable(cpuctx->ctx.pmu);
+next:
perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
}

local_irq_restore(flags);
}

-static inline void perf_cgroup_sched_switch(struct task_struct *task,
- struct task_struct *next)
-{
- struct perf_cgroup *cgrp1;
- struct perf_cgroup *cgrp2 = NULL;
-
- rcu_read_lock();
- /*
- * we come here when we know perf_cgroup_events > 0
- * we do not need to pass the ctx here because we know
- * we are holding the rcu lock
- */
- cgrp1 = perf_cgroup_from_task(task, NULL);
- cgrp2 = perf_cgroup_from_task(next, NULL);
-
- /*
- * only schedule out current cgroup events if we know
- * that we are switching to a different cgroup. Otherwise,
- * do no touch the cgroup events.
- */
- if (cgrp1 != cgrp2)
- perf_cgroup_switch(task);
-
- rcu_read_unlock();
-}
-
static int perf_cgroup_ensure_storage(struct perf_event *event,
struct cgroup_subsys_state *css)
{
@@ -1062,11 +1044,6 @@ static inline void update_cgrp_time_from
{
}

-static inline void perf_cgroup_sched_switch(struct task_struct *task,
- struct task_struct *next)
-{
-}
-
static inline int perf_cgroup_connect(pid_t pid, struct perf_event *event,
struct perf_event_attr *attr,
struct perf_event *group_leader)
@@ -1080,11 +1057,6 @@ perf_cgroup_set_timestamp(struct task_st
{
}

-static inline void
-perf_cgroup_sched_switch(struct task_struct *task, struct task_struct *next)
-{
-}
-
static inline u64 perf_cgroup_event_time(struct perf_event *event)
{
return 0;
@@ -1104,6 +1076,10 @@ static inline void
perf_cgroup_event_disable(struct perf_event *event, struct perf_event_context *ctx)
{
}
+
+static void perf_cgroup_switch(struct task_struct *task)
+{
+}
#endif

/*
@@ -3625,7 +3601,7 @@ void __perf_event_task_sched_out(struct
* cgroup event are system-wide mode only
*/
if (atomic_read(this_cpu_ptr(&perf_cgroup_events)))
- perf_cgroup_sched_switch(task, next);
+ perf_cgroup_switch(next);
}

/*